Carbon nanotubes are used for a variety of applications such as in sensors, reinforcement in composite materials, and the like. Although there are many different ways to form carbon nanotubes, there are two distinct types of carbon nanotubes, individual nanotubes formed by bulk synthesis and arrays of nanotubes formed by surface or oriented synthesis. The individual nanotubes are much like spaghetti where each nanotube is grown with random orientation. Arrays of aligned nanotubes may include literally billions of nanotubes side-by-side, formed on a substrate.
It has always been a goal to form longer and longer carbon nanotubes. If longer arrays of carbon nanotubes are formed, one can spin nanotubes into fibers that may be stronger and lighter than any existing fibers and that are electrically conductive.
The short length of carbon nanotubes has been a roadblock to many of their applications. Growth of carbon nanotubes with controlled morphology is an intensively investigated area. The ability to grow carbon nanotube arrays having a homogeneous and uniform structure over a large surface area greater than one square centimeter would enable carbon nanotube arrays to be used in many different structural and sensing applications.
Carbon nanotubes have been traditionally formed by chemical vapor deposition of carbon on a catalytic substrate. One effective catalytic substrate is iron. Nickel and cobalt have also been used successfully.
PCT Patent Publication 2008/105936 and US Patent Publication 2008/0095695, the disclosures of which are incorporated by reference in their entireties, disclose a catalyst composition for growing an array of aligned carbon nanotubes on a substrate having lengths greater than 1 millimeter up to and exceeding 2 centimeters, using a composite catalyst comprising a group VIII element and a non-catalytic element, preferably a lanthanide group metal. Exemplified are composite catalysts comprising iron (Fe) and a lanthanide group metal.
Notwithstanding the improvements in growing longer carbon nanotubes, a need remains for growing long, aligned arrays of carbon nanotubes that can be processed into carbon nanotube threads, yearns and other elongate structures.